Low Resistance Measurement

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What is the recommended procedure for measuring low resistances?
My DMM seems to wander all over the place depending on how I hold the
probes against the conductors. Is there a better method? It's
autoranging, so I don't know what range it's working with.
I seemed to have better results with my old, cheap, analogue meter.
jack

ps I'm looking at buying a cheap $90 megger for checking insulation
around the home and workshop. Any problems possible here?

 

[[email protected]]

As for problems with the megger, you could fry things if you don't know what
you are doing.

Thanks Tom, that's something I did not think of. I remember finding my
fridge unacceptable many years ago, but was told by a reliable source
that fridges are notoriously bad at current leakage.
Then I thought of my new fridge with its box of electronicals on top.
I won't check that! I was thinking of checking the windings of old
transformers that I am rewinding. No point wasting electricity melting
them if I've breached the insulation anywhere
Same goes for old motors that I might want to resurrect.
And then there is elderly house wiring...
jack

 

[[email protected]]

<[email protected]


** Use lotsa current.




** The analogue jobs use far more test current.

Ahh, thanks, Phil.
Come to daddy, li'l ol' analogue meter

The simplest way for the odd low R measurement is to use TWO meters at once.

One for current and the other for voltage.

Sort of how you can check earth wiring. Apply a decent load (electric
radiator eg) between active and earth and see if it works as per
normal.

Are there any DMMs out there that use a larger test current?

jack

 

[Tom Biasi]

What is the recommended procedure for measuring low resistances?
My DMM seems to wander all over the place depending on how I hold the
probes against the conductors. Is there a better method? It's
autoranging, so I don't know what range it's working with.
I seemed to have better results with my old, cheap, analogue meter.
jack

ps I'm looking at buying a cheap $90 megger for checking insulation
around the home and workshop. Any problems possible here?

Your DMM will certainly have a hard time with measurements below 1 Ohm.
For precise low ohm measurements a bridge meter designed for that purpose is
used.
As for problems with the megger, you could fry things if you don't know what
you are doing.

Tom

 

[Phil Allison]

<[email protected]

What is the recommended procedure for measuring low resistances?

** Use lotsa current.

My DMM seems to wander all over the place depending on how I hold the
probes against the conductors. Is there a better method? It's
autoranging, so I don't know what range it's working with.
I seemed to have better results with my old, cheap, analogue meter.
jack

** The analogue jobs use far more test current.

The simplest way for the odd low R measurement is to use TWO meters at once.

One for current and the other for voltage.





......... Phil

 

[Mike]

What is the recommended procedure for measuring low resistances?
My DMM seems to wander all over the place depending on how I hold the
probes against the conductors. Is there a better method? It's
autoranging, so I don't know what range it's working with.
I seemed to have better results with my old, cheap, analogue meter.
jack

ps I'm looking at buying a cheap $90 megger for checking insulation
around the home and workshop. Any problems possible here?

Your old analog meter seemed to better because it can't display the very small changes in the resistance of
the connections to the unknown resistance (contact resistance). The digital meter will show these variations
and as a result the reading will wander quite a bit. A much better method is what is called 4 wire or Kelvin
sensing. With this method 2 wires are used to supply a regulated constant current through the unknow
resistance so that the current flowing through the resistance is independent of the contact resistance. The
other 2 wires are used to measure the resulting voltage across the unknown resistance and that voltage is
scaled to represent the resistance. The resistance of the circuitry used to measure the voltage is very very
high compared to the resistance being measured so it has very little effect on the the voltage appearing
across the the unknown resistance. Some of the more expensive meters have this capability, but you can use
this method without such a meter by using a vaiable current regulated supply and setting the current through
the resistance and measuring the voltage across it then calculate the resistance. You can build a simple
current regulator pretty easiy just for this purpose.

Mike


"I think, therefore GOD is."
J.P. Moreland

 

[[email protected]]

** No.

Create your own.

Do NOT come back and ask ME how to do that.

Use your brain.

I didn't ask you anything, Herr Controller

jack

ps I can't honestly think of how to create one of these, but then I'm
just a newbie who has ventured onto your hallowed turf, apparently

 

[Phil Allison]

Are there any DMMs out there that use a larger test current?

** No.

Create your own.

Do NOT come back and ask ME how to do that.

Use your brain.



...... Phil

 

[[email protected]]

Your old analog meter seemed to better because it can't display the very small changes in the resistance of
the connections to the unknown resistance (contact resistance). The digital meter will show these variations
and as a result the reading will wander quite a bit. A much better method is what is called 4 wire or Kelvin
sensing. With this method 2 wires are used to supply a regulated constant current through the unknow
resistance so that the current flowing through the resistance is independent of the contact resistance. The
other 2 wires are used to measure the resulting voltage across the unknown resistance and that voltage is
scaled to represent the resistance. The resistance of the circuitry used to measure the voltage is very very
high compared to the resistance being measured so it has very little effect on the the voltage appearing
across the the unknown resistance. Some of the more expensive meters have this capability, but you can use
this method without such a meter by using a vaiable current regulated supply and setting the current through
the resistance and measuring the voltage across it then calculate the resistance. You can build a simple
current regulator pretty easiy just for this purpose.

Thanks so much for that, Mike.
So much more helpful than sending me away to "create" a DMM that uses
a higher test current. jack

ps I have a nice digital bench PS, so I will use that method to check
my transformer winding resistances.

 

[Phil Allison]

<[email protected]>



** Why don't you just.......

**** OFF TROLL !!

Before you electrocute your stupid self.







......... Phil

 

[Mike]

Thanks so much for that, Mike.
So much more helpful than sending me away to "create" a DMM that uses
a higher test current. jack

ps I have a nice digital bench PS, so I will use that method to check
my transformer winding resistances.

Here's an better description of Kelvin sense resistance measurment.
http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

You don't really need a lot of current to get reasonably accurate measurments.
You'll find that the winding resistance reading will creep upward as the current
you send through the winding heats up the wire, so it's best to use a little
current as you can and still get a decent voltage drop. You might start with 0.1A
so that your volt meter will read .1V/ohm. That way you have no math to do except
moving the decimal point. Hopefully your power supply will allow you to set
the current and not just the voltage. If so just set the current to 0.1A and the
voltage as low as you can and still maintain the 0.1A. Just be sure to measure the
voltage drop directly on the windings. I used to build and sell milliohm meters
to guys who race slot cars so they could check the windings on the little armatures.
I used 0.01A constant current and still that would warm the wire up enough to see
the readings creep up a bit. The Kelvin clips they show on the link above really
do make things a lot more convenient especially if you will doing a lot of measurments.

FWIW, phil allison and rest of his ilk are best totaly ignored. Those fools
troll around doing little more than trying to stir up trouble.

Your's was very reasonable question that deserves a reasonable response.
I'm no engineer, but I would be more than happy to lend any assistance that
I can to help you.

Mike

"I think, therefore GOD is."
J.P. Moreland

 

[Eeyore]

What is the recommended procedure for measuring low resistances?

I do it this way. I have a current limited PSU with an adjustable current limit
that I set to 1A. I then connect the item to be measured across the output
terminals and measure the volts across the item in question. Be sure to use a
Kelvin connection when measuring.

Graham

 

[Eeyore]

** No.

Create your own.

Do NOT come back and ask ME how to do that.

Use your brain.

That's asking a lot these days. Kids today are spoon fed their education mostly.

Graham

 

[[email protected]]

Here's an better description of Kelvin sense resistance measurment.
http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

Wonderful. Thanks. I've filed that as a keeper.

You don't really need a lot of current to get reasonably accurate measurments.
You'll find that the winding resistance reading will creep upward as the current
you send through the winding heats up the wire, so it's best to use a little
current as you can and still get a decent voltage drop. You might start with 0.1A
so that your volt meter will read .1V/ohm. That way you have no math to do except
moving the decimal point. Hopefully your power supply will allow you to set
the current and not just the voltage. If so just set the current to 0.1A and the
voltage as low as you can and still maintain the 0.1A. Just be sure to measure the
voltage drop directly on the windings. I used to build and sell milliohm meters
to guys who race slot cars so they could check the windings on the little armatures.
I used 0.01A constant current and still that would warm the wire up enough to see
the readings creep up a bit. The Kelvin clips they show on the link above really
do make things a lot more convenient especially if you will doing a lot of measurments.

I imagine that the test current used on my DMM is somewhat less than
100mA, and yes, my PS can set current. Problem solved. Thanks.

FWIW, phil allison and rest of his ilk are best totaly ignored. Those fools
troll around doing little more than trying to stir up trouble.

He apparently doesn't want me asking him certain questions, but I
understood that usenet allowed you to answer what you wanted and to
ignore what you were not interested in. Phil seems to have a
compulsion to answer everything and object when some of this
"everything" includes stuff that he doesn't want to answer. He needs
to understand just what he can change and what he can't. If he does
not like a question, then he would do himself a favour to just ignore
it, like every other sane member of this group has learned to do.
But perhaps he has a very empty life and usenet is a big part of it,
and so he wants it his way. Yep, your advice is sound. Thanks.

Your's was very reasonable question that deserves a reasonable response.
I'm no engineer, but I would be more than happy to lend any assistance that
I can to help you.

Thanks again, Mike, rgds... jack

 

[[email protected]]

That's asking a lot these days. Kids today are spoon fed their education mostly.

The same sentiments of 2000 years ago

jack

 

[[email protected]]

I do it this way. I have a current limited PSU with an adjustable current limit
that I set to 1A. I then connect the item to be measured across the output
terminals and measure the volts across the item in question. Be sure to use a
Kelvin connection when measuring.

Great advice, thanks, Graham.

jack

 

[Jamie]

What is the recommended procedure for measuring low resistances?
My DMM seems to wander all over the place depending on how I hold the
probes against the conductors. Is there a better method? It's
autoranging, so I don't know what range it's working with.
I seemed to have better results with my old, cheap, analogue meter.
jack

ps I'm looking at buying a cheap $90 megger for checking insulation
around the home and workshop. Any problems possible here?

If a megger is what you want so be it how ever, in case you
didn't already know. Most meggers generate HV in the average of
500 Volts. Others will do more depending on the range of test
you're doing.
for doing G/T ohms, our units at work can go up to 2500 Volts
to perform the test.

I guess if you were just doing test on transformers and things
of that nature, a basic meter of 500 volts will do fine.

I've seen some newer one's use lower voltages for test.

The general idea is to use a voltage on the fixture to which
the item under test was designed to handle. Also the HV makes it
easier to get High R readings.

 

[Phil Allison]

<[email protected]>


** Why don't you just.......

**** OFF TROLL !!

Before you electrocute your stupid self.

OTOH, that might be a positive outcome for humanity.



......... Phil